Direct Evidence of Heteroleptic Complexation in the Macroscopic Dynamics of Metallo-supramolecular Polymer Networks

Abstract

Heteroleptic complexes are widely employed in small-molecule supramolecular arrays for the construction of complex architectures or for inducing nanoscopic rearrangements upon application of external stimuli that change the coordination-geometry preference. Despite this potential, they are rarely employed in the development of metallo-supramolecular polymer networks; this is unfortunate as that strategy might actually provide a tool to build highly homogeneous model-type networks that could form a basis for both a myriad of elementary investigations on transient networks and for their use in rational soft-functional materials design. To close that gap, we mix aqueous solutions of terpyridine- and phenanthroline-functionalized tetra-arm poly­(ethylene glycol) (tetraPEG) precursors right at the overlap concentration and form physical hydrogels by introducing various types of divalent transition metal ions. The formation of heteroleptic complexes is comprehended by the persistence of the network percolation at intermediate network compositions, as revealed by rheological measurements. Specifically, shear stress curves and the resulting relaxation time spectra demonstrate the emergence of a third relaxation mode, on top of those associated with homoleptic complexes, which is indicative of heteroleptic complexation. Spectroscopic analyses and DFT calculations suggest the possible formation of heteroleptic complexes with all studied metal ions, whereby their fraction and lifetime are traceable using rheological measurements in the case of networks formed by Co2+ ions. Moreover, employing over-stoichiometric Co2+-to-ligand ratios eventually results in the selective formation of heteroleptic complexes. These results not only suggest new paradigms for devising smart soft materials, but they also propose new dimensions for characterizing heteroleptic complexes.

pH-Responsive Gelation in Metallo-Supramolecular Polymers Based on the Protic Pyridinedicarboxamide Ligand

“…Consequently, we study the rheological behavior of hydrogels under small-amplitude oscillatory shear (SAOS) deformation and follow the storage and loss moduli at different temperatures. The plateau modulus and the frequency at which the storage and loss moduli cross each other are directly determined by the equilibrium number and lifetime of transient cross-links, respectively. ,, As such, SAOS can provide a rich insight into the strength and stability of PDCA complexes. The slightly entangled solution of the polymer precursor does not demonstrate a viscosity noticeably larger than that of water .…”

“…Interestingly, the dynamic moduli could not be reproduced unless considering two very broad relaxation modes, which results in an apparent crossover at significantly low frequencies below 10 –5 rad s –1 , as shown in Figure S5a. This slow and broad relaxation can also be related to the contribution of the hindered polymeric relaxation of long chains in the presence of reversible supramolecular dynamics. − The dissociation of stable supramolecular bonds can be better understood by an exponential drop of shear stress in the stress relaxation experiment. , However, the corresponding shear stress curves only reflect a gradual power-law decay, as shown in Figure S6. This essentially limits the formation of a unique master curve by applying tTS and further suggests the remarkable stability of bis-PDCA–Co 2+ complexes and the broadness of the relaxation mode at low frequencies.…”

“…Among all of the transient bonds used, metal–ligand complexes are widely employed for the development of biomimetic transient hydrogels, not only because they can be formed simply in water but since their strength and stability can be freely tuned over several orders of magnitude by a mere change of the metal ion, its oxidation state, and the selection from the extensive library of ligands. − As such, the necessary coexistence of two relaxation modes with significantly different lifetimes to achieve self-healing can be realized simply by employing two different metal ions or two various ligands. ,,− In this regard, pyridinedicarboxamide (PDCA) is a unique tridentate ligand that is reported to form two distinct coordination bonds with significantly different dynamics upon complexation with the Fe 3+ ion. − Therefore, it has been widely integrated into different polymer chemistries and has shown promising self-healing properties. , Despite the acceptable individual performance, it has also been utilized in combination with carboxylate-based ligands, which are also capable of complexation with the Fe 3+ ion, and have shown synergistic properties. , The small-molecule variant of the PDCA ligand has shown a remarkable pH-sensitive complexation with covalent bond-like stability under basic conditions, which is a characteristic of protic ligands . Nevertheless, there is no information on the kinetics, thermodynamics, and pH responsivity of PDCA complexes, which otherwise could be employed to expand their utility as novel building blocks for metallo-supramolecular polymer networks.…”

Metal–ligand complexation and clustering in mussel-inspired side-chain functionalized supramolecular hydrogels

“…The stress relaxation experiment provides clear signatures of the relaxation processes, magnifies their temperature dependency, and makes it possible to extract different relaxation modes, which may overlap in frequency sweep experiments. 28–30 To analyze relaxation modes, the shear stress curve is fitted by eqn (1):The relaxation time spectrum, H ( τ ), can be expressed as a summation of N symmetric log-normal distribution functions, each of them corresponding to one individual relaxation mode:where i , w i , and PDI i represent the average relaxation time, its corresponding weight, and polydispersity index, respectively, for the relaxation mode i ; all should be considered as fitting parameters. The number of the relaxation modes is estimated following the analysis of the dynamic moduli.…”

“…The stress relaxation experiment provides clear signatures of the relaxation processes, magnifies their temperature dependency, and makes it possible to extract different relaxation modes, which may overlap in frequency sweep experiments. [28][29][30] To analyze relaxation modes, the shear stress curve is fitted by eqn (1):…”